Morphology of teleosaurid osteoderms from the Phu Kradung Formation of Thailand

Abstract

This is a morphological analysis of the osteoderm patterns of Indosinosuchus from the Late Jurassic Phu Noi excavation site in Kalasin Province, Thailand. Data on 203 specimens indicated that 197 osteoderms belongs to the teleosaurid genus Indosinosuchus. Out of 203 total, 109 specimens are dorsal osteoderm morphotypes (M1 to M5) with sub-square, sub-rectangular, or rhomboidal in shape, including keel and peg. Another 88 are ventral osteoderms (M6 and M7) with rectangular or sub-square osteoderms, obscure keel and peg. Large subcircular ellipsoid pits with an anterior facet bar are discovered in both dorsal and ventral osteoderms. Data on morphological features enabled osteoderm positions of Indosinosuchus to be described. The anterior cervical osteoderms are characterized by an obscure keel and a small sub-square shape (M1). In the posterior trunk region, the osteoderms increase in width and adopt a sub-rectangular shape (M2). These osteoderms decrease in size at the sacral level (M3) with a more pronounced keel. The osteoderms gradually decrease in size as they adopt a rhombus shape towards the caudal region (M4) until the caudal end (M5). The ventral osteoderms, with a lack of keel and peg, exhibit a rectangular shape in the central region (M6), while a square or sub-square shape is observed along its lateral margin (M7). Six specimens are could not be assigned to Indosinosuchus and were described as morphotype 8 (M8). This morphotype has distinct characteristics resembling the ventral osteoderms of Mesoeucrocodylia that are yet to be described. The discovery of Indosinosuchus and Mesoeucrocodylia osteoderms suggests a diverse range of crocodyliformes in the Phu Noi fossil excavation site with potential for future studies.